1. Field of the Invention
The present invention relates to a method for compensating critical dimension variations of patterns in photomasks. In particular, the present invention relates to a method for correcting the resultant critical dimension of a corresponding photomask in a given exposure procedure.
2. Description of the Prior Art
In integrated circuit manufacturing processes, a lithographic process is a mandatory technique. Accordingly, exposure techniques play an essential role in the semiconductor industry because advanced exposure techniques make the pre-determined patterns on the reticles easily and precisely transferred onto a substrate. In a lithographic process, a designed pattern, such as a circuit pattern or a contact hole pattern is created on one or several photomasks. The pattern on the photomask is then transferred by light exposure, using a stepper and scanner, into a photoresist layer on a semiconductor wafer. Only by using a lithographic process can a wafer producer precisely and clearly transfer a complicated circuit pattern onto a semiconductor wafer. It is an important issue to enhance the resolution of the lithographic process due to the reduced device sizes utilized in the modern semiconductor industry.
The specification of the exposure system determines the critical dimension of the pattern feature. Theoretically speaking, short wavelengths of light are desirable, as using shorter wavelengths of light to expose a photoresist layer will proportionally improve the resolution. The current IC processes require sub-micron resolution. Such sub-micron processes require that critical dimensions (CD) of the features be correctly and uniformly transferred onto the wafer with strict specifications.
In the current exposure system, the light source itself is the most basic factor that determines the critical dimension of the pattern feature. The shorter the wavelength of the light source is, usually the shorter the critical dimension of the pattern feature will be. With the trend of miniaturization of dynamic random access memory (DRAM), it is a key issue to enhance the critical dimension uniformity (CDU). For this reason, the critical dimension correction (CDC) has been proposed to solve the problems.
U.S. patent publication 2007/0065729 A1 proposes a method for compensating for critical dimension (CD) variations of pattern lines of a wafer, by the correcting the CD of the corresponding photomask. Ultrafast laser is employed to change the material property within the substrate of the product photomask in regions which correlate to regions of the wafer exposure field to result in the change in index of refraction, whereby attenuating light passing through the regions, so as to compensate critical dimension error for the CD variations on the wafer and hence provide an improved CD tolerance wafer.
However, such solution substantially changes the material property within the substrate and destroys the substrate of the product photomask. It is an irreversible process with some possible drawbacks and risks. First, once the product photomask is incorrectly changed, the only way to fix the problem is to replace the broken one with another new product photomask at an extra cost for the expensive new photomask and the delay of the production. Second, the change of the material property within the substrate of the product photomask in regions which correlates to regions of the wafer exposure field to result in the change in index of refraction may merely solve the problem of a specific instrument but can not generally solve the problem that different exposure instruments performs differently.
In the light of the aforesaid flawed solution to compensate critical dimension variations of patterns in photomasks, therefore, a novel technique is needed to provide a general, trans-instrument solution.